After my APC uninterruptable power supply (UPS) went dead, my home server did not boot anymore. After hooking it up a VGA monitor I saw the reason - the BIOS boot error:
UEFI defaults have been loaded.
The mainboard was a ASRock Q1900B-ITX, and I only bought the energy-efficient mini PC 1.5 years ago.
The BIOS still had the correct time and date, and after pressing F1 to continue the machine worked normally - so I thought it was a one-time glitch. But when shutting the machine down, unplugging and replugging it from electricity I saw the same error message again.
Replacing the CR2032 BIOS battery made the error go away.
The voltage the battery gave was probably too low to keep all of the data, so during bootup the BIOS probably noticed that they were corrupted and reset its internal state.